Ever more stringent emissions regulations have driven the compression ignition engine industry to adopt increased fuel injection pressures. One area of concern as a consequence of increased injection pressures relates to potential fatigue in the sac region of the fuel injector nozzle tip component. The sac region is often the thinnest pressure containment metallic layer, and also defines the nozzle outlets that extend between an interior volume of the fuel injector to the combustion space of the engine. The sac region will typically cycle through extreme high pressures with each engine cycle.
One strategy believed to have promise in strengthening fuel system components is to induce compressive residual stress on the inner surface the component. While a number of different strategies are possible for inducing compressive residual stress, an autofrettage process can be effective in inducing compressive residual stress on the interior surfaces of pressure vessels. For instance, Chapter 4 from Adis Basara's PhD. dissertation, Evaluation of High Pressure Components of Fuel Injection Systems Using Speckle Interferometry (2007), teaches sealing one end of a fuel line in order to perform an autofrettage process. Thus, an effective autofrettage process for a fuel injector nozzle tip may require that the nozzle outlets be sealed during the autofrettage pressurization procedure. Because the autofrettage pressures are so high, finding a robust production strategy for nozzle tips in a factory setting can be problematic.
Apart from problems associated with creating an effective autofrettage process for an injector tip, are problems associated with integrating the autofrettage process into the other steps associated with making a fuel injector tip. Typically, the raw shape of an injector tip will be made from steel rod stock. The shaping will include creating an inner surface that includes a conical needle valve seat and a sac region, from which the nozzle outlets extend to the outer surface of the tip. Because of the expected repeated impacts at the needle valve seat, it may be case hardened, such as via some known carbonization, carbo-nitraded or other case hardening process. However, case hardening a tip involves metallurgical heat treatment techniques that could tend to destroy compressive residual stress from an autofrettage process. In addition, the sac region of the fuel injector tip often also is in need of being strengthened to provide sufficient fatigue strength, as the sac region will undergo substantial fluid flow forces as well as cyclic pressure changes, and experience the most extreme temperatures of any part of a fuel injector. As a result of the case hardening techniques, the sac, needle valve seat, and other interior surfaces can achieve hardnesses in the range of HRC 55 or greater. However, the conventional wisdom is that such hardened surfaces are inappropriate for autofrettage, as one might expect the autofrettage process to produce micro-fractures or other problems in the hardened surface. Other strategies for inducing compressive residual stress, such as laser shock pending, could be considered but have their own problems, such as how to access all of the surfaces within the nozzle tip. For instance, the transition from the sac to the surfaces that define the nozzle outlets are a potential area where stress concentrations can occur, leading to fatigue crack development. Thus, the desire to induce compressive residual stress in certain surfaces within the fuel injector tip, may appear by conventional wisdom to be problematic with accepted manufacturing strategies for fuel injector tips.
The present disclosure is directed toward one or more of the problems set forth above.